[MooreToCore] Support for UnpackedArrayType emission

[if-sc]

Currently, CIRCT's MooreToCore pass converts unpacked arrays to packed arrays (hw::ArrayType),
losing the unpacked semantics.

When we've attempted to preserve unpacked arrays by emitting hw::UnpackedArrayType,
MLIR raises "failed to legalize operation" errors.

Minimal Reproduction

When running circt-opt on the following input,

module a (
  input logic b[3:0]
);
endmodule

we get the following result:

module {
  hw.module @a(in %b : !hw.array<4xi1>) {
    hw.output
  }
}

as you can clearly see, the uarray is converted to a array.

[fabianschuiki]

The errors you were seeing about legalization failure probably indicate that the lowering from HW/Comb to LLVM is missing a conversion pattern for the unpacked types. If you take a look at the ArcToLLVM lowering pass (used in the pipeline of arcilator.cpp), there is probably support for hw.array types, but not hw.uarray. Having lowering support for unpacked stuff would be fantastic!